Pigment-based inks potentially offer a variety of desirable properties, viz., waterfastness, lightfastness, etc. However, preparation of pigment-based inkjet inks has not been a widely successful approach due to tendency of pigments to agglomerate, which results in "precipitation" or "settling" of the colorants and a very short dispersion life. A variety of laborious and/or expensive approaches such as surface modifications of specific colorants, use of dispersants, and encapsulation of colorants in polymer coats, have been at the cutting edge of methods to alleviate these problems. These may result in loss of attributes such as waterfastness, increased cost and chemical complexity of the system, decreased material compatibility and limited choice of colorants. These methods do not share a common basis of stability. Therefore, inks based on these have to be individually developed for each colorant and dispersants and/or additives, each of which add new problems to the complexity of the ink. This results in enormous effort on ink-jet ink manufacturers to develop an ink system.
U.S. Pat. No. 5,169,438, issued on Dec. 8, 1992, to H. Matrick, discloses the preparation and use of water-insoluble colorants comprising pigments in ink-jet inks. The pigments are rendered soluble in the ink-jet ink vehicle by use of a dispersant, which keeps the pigment in suspension prior to printing on a print medium. The patentee discloses that the particle size of the pigment must be in the range of 0.005 to 15 .mu.m (5 to 15,000 nm).
No disclosure is given in this reference as to how to provide the necessary particle size of the pigment. However, it is common knowledge that such small particle sizes are achieved by ball milling or microfluidization. Ball milling involves milling the pigment particles in a ball mill with, typically, metal or ceramic balls, followed by size filtration, using a centrifuge or appropriate screening. However, a disadvantage with this process is that it introduces impurities from the balls during milling, which adversely affect the properties of the ink. Analogous grinding processes also suffer from the same defect.
What is needed is a pigment-based colorant that is stable in solution and is free of impurities, while employing a relatively facile process to prepare.